This invention relates to echo suppressors for two-way transmission systems and, more particularly, to improved break-in circuitry for echo suppressors.
Echos are produced in a transmission system whenever an impedance discontinuity or mismatch exists, such as at the junction between a four-wire transmission channel and a two-wire transmission channel. The annoying effects of echos can be reduced by the use of echo suppressor apparatus which essentially operates to disable the echo return path of a subscriber when he is transmitting. Thus, basically an echo suppressor is a voice-operated switching device which may be situated near one end of a four-wire transmission channel. If echos in both directions are to be suppressed by the device, it is commonly referred to as a full echo suppressor; if echos are suppressed in only one direction, the device is referred to as a split echo suppressor.
In a split echo suppressor, for example, the echo suppressor apparatus nearest a particular subscriber end typically functions to disable the transmit, or outgoing, path from that subscriber when signals from the far-end subscriber appear on the receive, or incoming, path. Thus, echos due to incoming signals on the receive path are prevented from returning to the far-end subscriber over the transmit path. Echo suppressor apparatus at the far subscriber end functions in a similar manner to prevent echos from returning to the near-end subscriber when the near-end subscriber is transmitting. A full echo suppressor essentially combines a pair of split echo suppressor essentially combines a pair of split echo suppressors in a single facility located at the near subscriber end, the two split echo suppressors sharing certain of the suppression control circuitry.
During echo suppression in a system utilizing either a full or a split echo suppressor, suppression must be removed from the transmit path when the near-end subscriber breaks in, a condition commonly referred to as double-talking since both subscribers are talking simultaneously. For this purpose, the echo suppressor includes break-in circuitry for distinguishing between speech signals generated on the transmit path by the near-end subscriber, which may be referred to as double-talking speech, and echo signals returning on the transmit path due to far-end subscriber speech signals on the receive path. This may be accomplished, for example, by sampling the signals on the transmit and receive paths and comparing the samples, or representations thereof, to determine their relative magnitudes. If the transmit path signals exceed the receive path signals, it is assumed that the near-end subscriber is transmitting and break-in is effected by removing echo suppression from the transmit path. On the other hand, if the transmit path signals do not exceed the receive path signals it is assumed that the near-end subscriber is not transmitting and the transmit path remains disabled. Similarly, in the case of a full echo suppressor, the break-in circuitry functions to remove suppression from the near-end receive path when the far-end subscriber breaks in.
A problem in distinguishing between echo and double-talking speech arises from the end delay of the echo in traveling from one transmission path to the other, e.g., from the receive path within the echo suppressor to the transmit path thereof. This end delay may be on the order of 25 milliseconds around the near subscriber end and, in the case of a full echo suppressor, up to 75 milliseconds around the far subscriber end. Thus, in known analog echo suppressors the samples from the one path are typically delayed, or stretched, to compensate for end delay before comparison with the samples from the echo return path. This may be accomplished by a simple lowpass RC filter arrangement, for example, and has been generally effective in suppressing echos. Similar digital arrangements have been proposed for use in digital echo suppressors. However, this approach does not produce a very accurate approximation of the anticipated echo and often results in an undesirable amount of clipping of a subscriber's speech signals, particularly if he speaks softly. Consequently, it would be desirable to improve the accuracy of the echo signal approximations which, in turn, would improve the speed and accuracy with which break-in can be effected so as to reduce excessive clipping of a subscriber's speech signals.